1. Field of the Invention
The present invention relates to antistatic footwear, and, more particularly, to antistatic footwear comprising an upper portion, an outer sole and an inner sole, also called insole, which is intended to overlap the sole side internal to the footwear and which is interposed between said outer sole and the user's foot. Electrical conductors passing through the sole provide for an electrical contact between the user foot and the ground.
2. Description of Related Art
Antistatic footwear is known and widely used. Although different types of antistatic footwear in the prior art accomplish their task satisfactorily, they also have some drawbacks.
Prior art antistatic footwear is often manufactured by embedding thin electrical conductors inside the sole, both in the foot tip region and in the heel region. These electrical conductors provide for a discharge to the ground of static electricity accumulated by a user. If the static electricity is not continuously discharged to the ground, the user may be affected by unpleasant shocks, for example when he touches metallic parts having a ground or earth electrical contact.
Prior art footwear has some drawbacks. Electrical conductors disposed in electrical contact with the user body and with the ground, for discharging the static electricity accumulated on the user, are often buried into rubber forming the sole. This entails very high manufacturing costs, since during manufacturing it is first necessary to prepare the antistatic conductor or thread interlacement and subsequently to perform the rubber casting.
Moreover, prior art antistatic soles typically include antistatic conductors or threads made of a metallic material. Metal threads or conductors involve a very low electrical resistance and, therefore, the risk is present that an electric charge on the ground is discharged on the user, causing a potential trouble situation and even a health danger if the voltage on the ground is sufficiently high for causing a step current, or a current passing through the user body for known reasons when the voltage between the feet of the user reaches a certain level. Typically, the step current is generated when the user is near an electrical cable that is in electrical contact with the ground because of an accident. Metal materials disposed inside conductors or threads of prior art soles may easily conduct step currents through the body of the user because of its low electrical resistance.
A further problem in the prior art is that antistatic threads or conductors are often uniformly arranged under the foot or on the sole surface. However, the foot does not rest completely and uniformly on the ground, and moreover during walking the foot exerts a pressure on the ground that is not uniform on all the surface of the support: there are indeed regions of support of the foot that are subject to a greater pressure and foot regions that are hardly submitted to the action of body weight and, therefore, that are not subject to weight pressure.
Since it is known that electrical conduction is highly dependent on the pressure between two bodies in contact one with the other, a person skilled in the art will understand that not all of the foot sole is in contact with the sole and so not all of the sole has the same electrical conductivity. This leads to the inevitable drawback of a different resistance in different portions of the foot sole and particularly there to a greater resistivity in foot regions submitted to a lower pressure. As a result, since electrical current flows always in the point of lowest resistance of a circuit, the electrostatic charge discharging from the foot to the ground preferably passes through determined foot regions, that is, those submitted to a greater pressure. Due to this phenomenon, small electrostatic charges are perceptible by the user when discharged to the ground.
The present invention aims at providing antistatic footwear of the type above described, in which drawbacks of known antistatic soles are overcome in a simple and inexpensive manner, while allowing for simpler and less expensive footwear manufacturing processes.